Scotland's top prosecutor has reaffirmed Abdelbaset al-Megrahi's guilt in the killing of 270 people in the Lockerbie bombing and has pledged to track down his accomplices. Lord Advocate Frank Mulholland said no Crown Office investigator or prosecutor has raised a concern about the evidence in the bombing of Pan Am flight 103 26 years ago.

Al-Megrahi's part in the bombing has been called into question in a series of books, documentaries and testimony to the Scottish Parliament. A petition seeking "Justice For Megrahi", backed by politicians and family members of some victims, remains on Holyrood's books two years after al-Megrahi's death.

But in an anniversary address to relatives, Mulholland will say his investigation "remains on the evidence, and not on speculation and supposition". He has travelled to Washington to attend a memorial service at Arlington cemetery in Washington on Sunday.

Mulholland will say: "The current instability in Libya has meant that some investigative opportunities have required to be reassessed, which I know has been frustrating for family members. However our prosecutors and police officers, working with UK Government and US colleagues, will continue to pursue this investigation, with the sole aim of bringing those who acted along with Al Megrahi to justice.

"There are other significant investigative opportunities open to us which are not reliant on obtaining evidence from our Libyan colleagues. The Crown will never give up the fight to secure justice for the families of those who died.

"It might be 26 years since 270 people lost their lives in the terrorist attack but justice has no sell-by date in Scotland. Over the years many people have worked on the inquiry and all have been given the same instruction; to carefully review the evidence and work to identify all of those who were involved in the conspiracy to destroy Pan Am flight 103.

"During the 26-year long inquiry not one Crown Office investigator or prosecutor has raised a concern about the evidence in this case. We remain committed to this investigation and our focus remains on the evidence, and not on speculation and supposition."

Former Liberal Democrat leader Sir Menzies Campbell said: "One of the most unsavoury elements of this case is the repeated and unfounded criticism of the judges who sat in the trial, and those who heard the subsequent appeal against conviction.

"The judges by convention are unable to respond publicly to these criticisms, which imply that they were somehow part of a conspiracy. In truth, they were all experienced in criminal law and of unquestioned integrity. It is a curious feature of this case that those who argued most vehemently for a special court to be set up to deal with the case are now among the most vociferous critics of its verdict."

After being found guilty, Al-Megrahi was sentenced to life before being released in 2009 as he was dying of terminal cancer.